Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
In many wind turbines, the generator may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. Such wind turbine power systems are generally referred to as a doubly-fed induction generator (DFIG). DFIG operation is typically characterized in that the rotor circuit is supplied with current from a current-regulated power converter. As such, the wind turbine produces variable mechanical torque due to variable wind speeds and the power converter ensures this torque is converted into an electrical output at the same frequency of the grid.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally coupled to the generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator. Rotational energy is converted into electrical energy through electromagnetic fields coupling the rotor and the stator, which is supplied to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
Wind turbines that utilize DFIGs can produce flicker due to design and/or manufacturing variations. More specifically, as the generator rotates, differences in poles and/or one or more phases can cause variations in the stator reactive current. At certain speeds, the variations can be of a frequency in the range defined as flicker. The term “flicker,” as described herein, generally refers to variations in current or voltage that are perceptible at certain frequencies (e.g. from about 1 Hertz (Hz) to about 30 Hz). Flicker may also be caused by radial variations in the air gap of the generator, for instance due to saliency in the rotor design and/or construction. As used herein, a salient pole-type of rotor has of large number of projected poles (often referred to as salient poles) mounted on a magnetic wheel, in contrast to non-salient pole rotors that have a cylindrical shape with parallel slots thereon to place rotor windings. Oftentimes, grid requirements prohibit connection to the power grid if flicker is present in a certain amount.
Thus, the present disclosure is directed to a system and method for compensating for generator-induced flicker in wind turbines connected to the power grid so as to address the aforementioned issues.